Fluid separation techniques are, for example, applied for brackish or seawater desalination. In order to obtain potable water, for example, membrane-assisted distillation techniques are conventionally used. Known membrane-assisted techniques include reverse osmosis, electrodialysis or membrane distillation. In membrane distillation processes, phase change of the liquid feed takes place at the surface of a hydrophobic membrane. The membrane is a barrier for the liquid phase of the feed water, but permeable for the vapor phase, for example water vapor. The driving force of the process is given by a partial vapor pressure difference, for example by applying a temperature difference or gradient between the two sizes of membrane. Due to the temperature difference and the phase change, considerable thermal energy has to be employed for the operation of a membrane distillation process.
For example, US 2009/0000939 A1 discloses a membrane distillation process in which a liquid to be concentrated is separated from the vapor space by a vapor-permeable but liquid or water-impermeable membrane. In this method, a negative pressure is imparted to the liquid to be concentrated so that the absolute pressure is reduced to or below its boiling vapor pressure corresponding to the temperature. The corresponding membrane distillation device has several stages including condenser modules.
Because membrane distillation processes and devices are less efficient than other desalination processes, it is desirable to reduce their total thermal energy consumption, when operating such a membrane desalination system.